This invention relates generally to systems and methods for sensing levels or quantities of substances in containers, and more particularly, to sensing liquid levels using capacitive coupling.
There are numerous applications that require the sensing of a liquid level in a container, and numerous methods for sensing the level or amount of the liquid in a container. Primitive techniques use a dip-stick. Many other containers incorporate floats coupled to a variable resistor. Photodiode sensors can also be used with opaque liquids and transparent containers.
One whole class of techniques use capacitive plate type immersion, see U.S. Pat. Nos. 4,448,071, 5,270,210, and 4,589,077. The problem with immersion type sensing is that the xe2x80x9cprobexe2x80x9d is in contact with the liquid. This may not be desirable in applications where contamination of the liquid must be avoided, e.g., in toxic, radioactive, or biological applications, and for consumable goods. Also, in all of these techniques, the probe is wired to sensing circuits. This makes it difficult to move the container around, or to carry the container from one location to another.
Therefore, it is desired to sense the level of a liquid without direct contact with the liquid, and, in addition, it is desired that the sensing can be performed via a wireless connection to provide for a portable container.
The invention provides a wireless liquid sensing system that includes a beverage container and a table top for holding the container. Embedded in the walls and bottom of the container, for example a water glass, are two electrically conductive plates coupled to a transponder wire coil.
A wireless reader, embedded in the table top, radiates an RF signal at a predetermined frequency through a reader antenna. A microprocessor in the container, also coupled to the transponder coil and the two plates, is powered by a rectifier circuit that gains power from then radiated RF signal. The microprocessor amplitude modulates the RF signal in accordance with the amount of liquid in the container. The reader can then detect this modulation with a peak detector to sense the amount of the substance in the container when the transponder antenna is inductively coupled to the reader antenna at the predetermined frequency. The modulation can be done by loading the transponder coil.